Description
Introduction
The objective of this laboratory is to provide you with practical experience working with interrupts/exceptions. Interrupt
handling is an extremely important and fundamental aspect of how a microprocessor interacts with the peripheral
components to which it is attached. It provides an efficient way for a peripheral to signal to the processor that it requires
the processor’s attention. Te processor can then pause what it is currently doing and quickly respond to the peripheral’s
request for service.
In this laboratory you will again begin with a supplied example project and modify its operation. Create a new ARM
assembly language project and select the Interrupt Example sample program. Note that when you get to the last screen of
the project setup, the Memory options selection will be set to Exceptions. (Tis setting is pre-selected and can’t be changed.)
You can see from this setup screen that the memory locations used by the ARM processor for its interrupt vector table is
defned under the .vectors label.
When you build, download, and run the Interrupt Example program you will see two things happening. Te frst is that
the red LEDs on the board will strobe lef or right. Te direction the LEDs move may be toggled by pressing either KEY0
or KEY1. Te second activity is that the green LED near the bottom right corner of the board will blink at a frequency of
2 Hz. Tis example program is actually using three different interrupt events to accomplish this seemingly simple
behaviour.
Te simplest interrupt event is a reaction to a KEY button being pressed. In the past you actively polled the state of the
buttons by reading the data register for the GPIO port to which the KEY buttons are connected. In this program, the
GPIO port is setup to generate an interrupt when a KEY is pressed and released. It is actually the release of the button
which triggers the interrupt request, and the button state is captured and saved at that time. In response to the KEY
interrupt, the processor changes the direction in which the LEDs are moving.
Te other two interrupts in this example are generated by two separate timers. Te timers are simply counters which
count down from a starting value (setup at the beginning of the program) to zero. When they reach zero, an interrupt is
generated, and the counter is automatically reset to the same starting value and begins counting again. One of these
timers, called the Interval Timer, is setup to produce an interrupt every 50ms. Tat interrupt signals to the processor that it
should shif the pattern on the red LEDs over by one position. Similarly, the second timer, called the HPS Timer, is setup
to produce an interrupt every second. Tat interrupt indicates to the processor that it should toggle the state of the green
LED.
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ECE 3610 Microprocessing Systems Laboratory 5
To keep the different aspects of this program organized, the project is broken up into eight separate program fles. A brief
description of each is provided in the following table.
Procedure
Create, build, and run the Interrupt Example program. Observe its operation, and how the system responds to the KEY
press. Notice that the red LEDs change direction when the KEY is released, not when it is pressed. Also notice that the
fashing of the green LED is not affected by pressing or holding the KEY buttons, but operates completely independently.
Both the timer and the key interrupts are all in the IRQ interrupt category. When any of those interrupts occur, the
processor will follow the IRQ vector in the interrupt vector table (at memory location 0x00000018). Tat vector is a
branch to the SERVICE_IRQ routine located in the exception.s fle. Tat routine frst saves part of the processor state
by pushing registers {R0–R7,LR} onto the stack. It then checks for the source of the interrupt by reading the interrupt ID
number from the interrupt controller. Tis ID number tells the processor which peripheral triggered the IRQ interrupt. It
then branches to a different service routine for each of the three interrupt sources: KEY_ISR, TIMER_ISR (for the interval
timer), or HPS_TIMER_ISR. When the ISR ends, it branches back to SERVICE_IRQ which clears the interrupt, and
restores the original processor state by popping {R0–R7,LR} back off the stack before returning from the interrupt.
Examine each of the three ISRs to familiarize yourself with their general operation.
Modifying the Program
1. Te example program is setup to respond to KEY0 or KEY1, but ignores KEY2 and KEY3. Tis is controlled by the
setting of an interrupt mask register for that specifc GPIO port. Currently the mask value is set to 0x03 in the
CONFIG_KEYS section of the interrupt_example.s fle. Changing this value to 0x0F will enable interrupts for any
of the KEY buttons.
Modify the KEY_ISR and TIMER_ISR routines as needed so that KEY0 sets the LED direction to lef-to-right (instead
of toggling it), while KEY1 sets the direction to right-to-lef.
2. Modify the program further, so that KEY3 stops the red LEDs from cycling, while KEY2 starts the red LED cycling
again. Note, do not change the timer settings to achieve the stated behaviour, but rather modify TIMER_ISR (and any
other parts of the program you deem necessary) so it does not update the LEDs when in the stopped state.
File Name Description
interrupt_example.s Main program fle that performs setup and contains the main program loop.
exceptions.s Defnes the service routines for the different types of exceptions that might occur.
Te three interrupts used in this program are all of the IRQ type.
hps_timer_isr.s Te interrupt service routine for the HPS Timer (which controls the green LED).
key_isr.s Te interrupt service routine for the KEY buttons.
interval_timer_isr.s Te interrupt service routine for the Interval Timer (which controls the red LEDs).
defines.s General .equ defnitions used in this program.
interrupt_IDs.s Te list of identifers used to distinguish between the different sources of the IRQ interrupt.
address_map_arm.s List of addresses where different hardware components are memory mapped.
(Tis same fle has appeared in all the example projects.)
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ECE 3610 Microprocessing Systems Laboratory 5
3. For the green LED, the HPS_TIMER_ISR code currently increments a location in memory, labelled TICK, each time
the HPS Timer generates an interrupt. Te mainline program (in interrupt_example.s) continuously checks the
TICK memory location to see if it is zero or non-zero. If zero, then it does nothing but check again. When the TICK
location becomes non-zero, it is the mainline that toggles the state of the green LED. It also sets the TICK value back
to zero again. Modify the mainline and the HPS_TIMER_ISR code so that the state of green LED is toggled directly by
the HPS_TIMER_ISR, not in the mainline.
